1. Field of the Invention
The present invention relates generally to a zoom lens and, more particularly, to an inner focus zoom lens suited to an autofocus camera.
2. Related Background Art
There have hitherto been proposed a multiplicity of zoom lenses each including a first negative lens unit, a second positive lens unit, a third negative lens unit and a fourth positive lens unit or including a first positive lens unit, a second negative lens unit, a third positive lens unit and a fourth positive lens unit. The great majority of such zoom lenses adopt a so-called first lens unit extension focusing system in which focusing is conducted by moving the first lens unit.
When the zoom lens based on the conventional first lens unit extension focusing system is mounted in an autofocus camera, the first lens unit, which is the largest and heaviest lens unit, is driven by a motor, and, therefore, the focusing is hard to speed up. Under such circumstances, the focusing performed by a lens unit exclusive of the first lens unit was proposed in Japanese Patent Publication No. 5-67930 and Japanese Patent Application Laid-Open Nos. 2-296208, 4-235514, 5-107476, 5-173070, 5-173071, 57-5012, 56-21112 and 58-143312.
The zoom lens in the second embodiment in Japanese Patent Publication No. 5-67930 is constructed of a first negative lens unit, a second positive lens unit, a third negative lens unit and a fourth positive lens unit, wherein the focusing is effected by moving the third lens unit. There arises, however, a problem wherein a focusing moving quantity is 5.285 mm with respect to a photographing distance of 1.5 m and therefore reaches substantially 10% of a focal length (49 mm) at the telephoto end, and the zoom lens increases in size to secure a moving space of the focusing lens unit. Another problem is that the image forming magnification of the third lens unit for focusing is required to be in the vicinity of -1 at the wide angle end, and a degree of freedom of a power construction is restricted, resulting in a difficulty in terms of designing.
The zoom lens in the embodiment in the Japanese Patent Application Laid-Open No. 2-296208 is constructed of a first negative lens unit, a second positive lens unit, a third negative lens unit and a fourth positive lens unit. The first lens unit is divided into two lens subunits, and the closer-to-image-surface lens subunit of these subunits is moved, thus performing the focusing. However, the first lens unit is divided into the two lens subunits, and hence the zoom lens is constructed of substantially five lens units. Therefore, the problem is an increase in costs with a rise in the number of the lens units.
The zoom lens in the embodiment in the Japanese Patent Application Laid-Open No. 4-235514 is constructed of a first negative lens unit, a second positive lens unit, a third negative lens unit and a fourth positive lens unit. The focusing is effected by moving the third lens unit or the third lens unit and the fourth lens unit. However, the focusing moving quantity with respect to the same photographing distance largely differs depending on the focal length, and there arises a problem of causing defocus when the zooming is carried out after performing the focusing operation.
The zoom lens in the embodiment in the Japanese Patent Application Laid-Open No. 5-107476 is constructed of a first negative lens unit, a second positive lens unit, a third negative lens unit, a fourth positive lens unit and a fifth positive lens unit. The focusing is effected by moving the fifth lens unit. As in the case of Japanese Patent Application Laid-Open No. 4-235514, however, the focusing moving quantity with respect to the same photographing distance largely differs depending on the focal length, and there also arises the problem of causing defocus when the zooming is carried out after performing the focusing operation.
The zoom lens in each embodiment in the Japanese Patent Application Laid-Open Nos. 5-173070 and 5-173071 is constructed of a first negative lens unit, a second positive lens unit, a third negative lens unit and a fourth positive lens unit. The second lens unit is divided into two lens subunits, and the closer-to-object lens subunit of these subunits is moved, thus performing the focusing. However, the second lens unit is divided into the two lens subunits, and, therefore, the zoom lens is constructed of substantially five lens units. Accordingly, the problem is the increase in costs with the rise in the number of the lens units.
The zoom lens in the embodiment in the Japanese Patent Application Laid-Open No. 57-5012 is constructed of a first positive lens unit, a second negative lens unit, a third positive lens unit and a fourth positive lens unit. The focusing is executed by moving the fourth lens unit in the optical-axis direction. However, the focusing moving quantity with respect to the same photographing distance largely differs depending on the focal length, and there arises the problem of causing the defocus when the zooming is carried out after performing the focusing operation.
The zoom lens in the embodiment in Japanese Patent Application Laid-Open No. 56-21112 is constructed of a first positive lens unit, a second negative lens unit, third negative lens unit and a fourth positive lens unit. An afocal air spacing is formed within the fourth lens unit, and the focusing is carried out by integrally moving the lens elements from the second lens unit up to the portion of the fourth lens unit, adjacent to the object side of the afocal air spacing in the optical-axis direction. According to this focusing method, the focusing moving quantity with respect to the same photographing distance is fixed without depending on the focal length. The first lens unit and the fourth lens unit are, however, fixed during the zooming, and, hence, it is difficult to reduce an entire length of the zoom lens.
The zoom lens in the embodiment of Japanese Patent Application Laid-Open No. 58-143312 is constructed of a first positive lens unit, a second negative lens unit, a third positive lens unit and a fourth positive lens unit. The focusing is conducted by integrally moving the third lens unit and the fourth lens unit in the optical-axis direction. According to this focusing method, the zoom lens is constructed so that a relational expression (fA..beta.F) / (.beta.F.sup.2 -1).sup.1/2 between the combined focal length fA of the closer-to-object lens unit than the focusing lens unit and the image forming magnification .beta.F of the focusing lens unit is substantially fixed during the zooming. The focusing moving quantity with respect to the same photographing distance can be thereby substantially fixed without depending on the focal length.
However, there exists a problem in which the degree of freedom of the power construction is limited in terms of a constraint of satisfying this relational expression, resulting in a difficulty of designing.